In fiber optic communications, dense wavelength division multiplexing (DWDM) is a technique for multiplexing multiple optical carrier signals onto a single optical fiber. This form of frequency division multiplexing is commonly referred to as DWDM when applied to optical systems that employ a high level of multiplexing. The potential of optical fiber is more fully exploited when multiple beams of light at different frequencies (wavelengths) are transmitted on the same fiber. By using different wavelengths of laser light to carry different signals, capacity is multiplied. In a DWDM system, a multiplexer is used at the transmitter to join the signals together and a de-multiplexer is used at the receiver to split the signals apart.
An optical ring resonator is a device that is capable of both multiplexing and de-multiplexing, and it can function as an add-drop multiplexer on a fiber-optic communication bus. Optical ring resonators include a waveguide in a closed loop, coupled to one or more input/output (or bus) waveguides. When light of the appropriate wavelength is coupled from an input waveguide to the ring, constructive interference causes a buildup in intensity over multiple round-trips through the ring. The light is ultimately coupled to an output waveguide. Since only selected wavelengths resonate in the ring, the ring functions as a filter. A range of applications such as optical switching, electro-optical switching, wavelength conversion, and filtering have been demonstrated using optical ring resonators.
Conventional multiplexer switches integrated with wavelength selective devices, such as micro-electromechanical mirrors (MEMS), liquid crystal mirrors or lenses, and thermo-optic switches are typically limited to switching speeds in the millisecond range; even the fastest response ferroelectric liquid crystals and smallest thermo-optic switching speeds are limited to several microseconds. Although these components are suitable for configurable DWDM circuit networks, they are not suitable to replace electrical cross-connect switches used for network applications requiring very rapid (e.g., less than one microsecond) switching speeds, such as applications for tactical aircraft avionics. These relatively slow switches are also unsuitable for use in DWDM optical burst or optical packet switch based networks.
An optical add-drop multiplexer is a device used in DWDM systems for multiplexing and routing different channels of light into or out of an optical fiber. An optical add-drop multiplexer with remotely reconfigurable optical switches in the middle stage is called a Reconfigurable Optical Add-Drop Multiplexer (ROADM). A ROADM has the ability to remotely switch traffic from a DWDM system at the wavelength layer.
Current technology for ROADM devices: (1) switch too slowly; (2) only switch over a very limited wavelength range, namely a single full width at half maximum (FWHM); (3) are not readily compatible with silicon processing; (4) do not readily scale to compact wavelength selective cross-connect arrays with a large number of fibers=N, and wavelengths=M; (5) are limited to configurable DWDM circuits; (6) do not readily extend over a 20 nanometer free spectral range required for wavelength switching across the C-band; and (7) do not operate at practical voltages since electro-optic coefficients are insufficient and electrode gaps are too large.
Moreover, Mach-Zehnder (MZ) interferometric switches, movable micro-mechanical systems (MEMS) mirror arrays, tunable glass MZ array waveguide gratings (AWGs), 2×2 digital optical silica waveguide switches, 1×2 liquid crystal switches, bubble switches, 2×2 electro-holographic switches can be relatively large and typically only switch a single DWDM ITU grid (e.g., 25, 50, 100 GHz channel spacing) over no more than one full width half maximum (FWHM). This restricts device design to 1×2 or 2×2 switches where a wavelength is switched between one of two physical optical paths. The number of devices required to fabricate a large N-fiber by M-wavelength ROADM or cross-connect increases as the product of N by M. For non-blocking behavior, even larger cascaded, cross-connect arrays are required. Large ROADM or wavelength selective cross-connect arrays based on ceramic electro-optic 2×2 digital waveguide devices are not scalable due the large array areas resulting for the large number of cm size devices required, and the need to minimize cross-talk and insertion losses due to cross-overs.